1. Field of the Invention
This invention relates in general to a method and an apparatus for growing a single crystal according to the Czochralski method, and more particularly to a method and an apparatus for feeding the granular raw material.
2. The prior Art
When manufacturing a silicon single crystal according to the Czochralski method, the granular raw material, i.e. silicon poylcrystals, which is the raw material for the single crystal is put into a crucible and heated above the melting point of silicon to obtain a melt. A seed crystal is then dipped in the melt, and then pulled upwardly to grow a single crystal rod. The amount of the melt in the crucible decreases as the pulling process progresses, so that it is required to make up for the decrease of the melt by feeding the granular raw material to the crucible. An apparatus which have a granular raw material feeding pipe is used for feeding the granular raw material.
One of these methods for feeding the granular raw material using such an apparatus is so-called continuous charging method in which the granular raw material is continuously feeded onto the surface of the melt in the crucible while the single crystal is grown. FIG. 4 shows an example of the configuration of the single crystal pulling apparatus and the granular raw material feeder used for growing a single crystal according to the continuous charging method. In FIG. 4, the single crystal pulling apparatus 21 has a crucible which is not shown, and the granular raw material is fed into the crucible from the granular raw material feeder 30, whose configuration is enclosed by dotted lines.
The granular raw material feeder 30 comprises a feeding reservoir 32 which is connected to the single crystal pulling apparatus 21 through a gate valve 31, a gas feeding section 35 which is interconnected to the feeding reservoir 32 through a valve 34 and supplies argon and such, and a vacuum pump 36 which is interconnected to the feeding reservoir 32 through a valve 33. The feeding reservoir 32 has a granular raw material supplying section 38 by which the granular raw material is supplied through a valve (or an opening which can be sealed) 37. The pressure in the feeding reservoir 32 is always kept at the same pressure as the inner pressure of the single crystal pulling apparatus 21 (typically 10-1,000 hPa) and it is possible to feed the granular raw material into the single crystal pulling apparatus 21 through the gate valve 31 while the single crystal is being pulled. The valves 33, 34 and 37, or all valves except the gate valve 31, are closed during feeding.
For supplying the granular raw material to the feeding reservoir 32 when the inner pressure of the single crystal pulling apparatus 21 is reduced, the gate valve 31 is closed firstly, then the valve 37 is then opened to restore the inner pressure of the feeding reservoir 32 back to the atmospheric pressure. Then the feeding reservoir 32 is removed, and the granular raw material is directly supplied to the feeding reservoir 32 through the granular raw material supplying section (loading port) 38. An alternative method involves setting up a drum can containing the raw material or an intermediate feeding reservoir on top of the feeding reservoir 32, connecting it to the granular raw material supplying section 38 and then pouring the granular raw material through the valve 37.
After supplying the granular raw material, the feeding reservoir 32 is replaced if it has been removed, the valves 34 and 37 are closed, then the valve 33 is opened with the vacuum pump 36 running to establish a vacuum in the feeding reservoir 32, and then the valve 33 is closed. Then the valve 34 is opened to supply argon from the gas feeding section 35 into the feeding reservoir 32 so that the inside of the feeding reservoir 32 is filled with argon gas whose pressure is the same as the pressure in the single crystal pulling apparatus 21. Then the valve 34 and the gate valve 31 are closed one by one. By the processes shown above, it is possible to feed the granular raw material into the single crystal pulling apparatus 21.
The continuous charging method as described above makes it possible to grow a longer single crystal rod limited by the capacity of the feeding reservoir, and therefore allows a substantial improvement in the manufacturing yield and a drastic reduction of the manufacturing cost.
In addition to the continuous charging method described above, the multiple-pulling method, a version of the conventional batch process, is also known as a method of reducing the manufacturing cost (refer to Semiconductor Silicon Crystal Technology, Fumino Shimura, pp178-179, 1989). In this method, after pulling a single crystal with a dopant concentration in the range which satisfies prescribed resistivity standards, additional charging (hereafter referred to as recharging) of the granular raw material in the amount equal to the weight of the grown single crystal rod is carried out by the batch process, and another rod of single crystal is pulled again. By this way, a plurality of single crystal rods can be manufactured from the melt once loaded. Therefore this method makes it possible to improve the manufacturing yield and reduce cost for the crucible. In this multiple-pulling method, the granular raw material is also fed from the feeding reservoir while recharging is being carried out.
In the conventional method for feeding granular raw material as described above, continuous charging and recharging of the granular raw material is impossible while feeding the granular raw material into the feeding reservoir during the pulling process, in which the gate valve between the feeding reservoir and the single crystal pulling apparatus is closed, as described before, and the inner pressure of the feeding reservoir is restored to the atmospheric pressure before feeding the granular raw material. Therefore it results in poor efficiency.